I am looking for one word that is explained as "the belief in the good nature of humankind".
It is not philanthropy or humanitarianism; rather, I am looking for one word to describe one's belief in philanthropists.

The term comes from a character in Voltaire's Candide, which "begins with a young man, Candide, who is living a sheltered life in an Edenic paradise and being indoctrinated with Leibnizian optimism (or simply Optimism) by his mentor, Pangloss."

■ It would be a dangerous and biased way to use "humanism" in that case. Humanism is mainly define by the fact of giving price to human life. If you attach the concept of humanism to the belief that human nature is innately "good", then you threaten the core value of humanism (this belief about human nature has been badly shaken by science since the 80's). Giving price to life doesn't imply the need of a "good" human nature (cf. the is–ought problem). The same apply to "benevolence" as it could merely be a learning and not an innate state.

■ "Naivety" or "optimism" are subjective and thus are more interpretations than definition of "belief in the good nature of humankind”

I suggest "Rousseauian" (like JeffSahol) which is based on the "Noble savage" belief:

Rousseau believed that "nothing is more gentle than man in his primitive state".